One known method of marking sheet articles is to use, for example, paper sheets comprising small amounts of very thin metallic fibres distributed in the sheets and capable of absorbing and reflecting certain proportions of the energy of microwave radiation impinging on them. Therefore, it has been considered to apply this property to, among other things, certain types of security paper, such as banknotes, passports and bonds for their identification or verification of their authenticity by traversing them on a support through the path of a microwave emitter device and by detecting and measuring the proportion of the energy reflected and absorbed by the electrically conductive fibres embedded in the sheets. The amount of conductive fibres must remain small so as not to change too much the aspect and properties of the sheets or plates. It has been described in the French Patent Application No. 80,09095 of Applicant that fibres with smooth surfaces possessing a conductivity inferior to 10% of that of the standard of copper, with a diameter inferior to 50 .mu.m and a length of less than 10 mm, produce an excellent microwave identification signal at quantities of e.g. 0.5% by weight in the article. These fibres produce a specific response which can hardly be imitated by other materials so that counterfeiting the marking is prevented. Moreover, it is generally desirable that the microwave detection device be very sensitive and capable of rapid response, and permit reproducible identifications of the same article.